1. Field of the Invention
The present invention relates to a polysulfone-based resin solution composition and laminate using the same, more particularly a polysulfone-based resin solution composition which forms a smooth coating layer when spread on a base surface, and does not generate any harmful component, and a laminate which is produced by spreading and drying the same polysulfone-based resin solution composition on a base surface.
The laminate has sufficiently excellent optical and surface characteristics to be used for a heat-resistant optical film or the like.
2. Description of the Prior Art
A polysulfone-based resin is excellent in various properties, e.g., heat resistance, dimensional stability, mechanical characteristics, creep resistance, flame retardancy, safety to humans, and acid and alkali resistance. Moreover, it has a very high solubility parameter (SP value) and peculiar properties. As such, the molded articles of the resin have been widely used in various industrial areas, e.g., electric/electronic devices (e.g., printed circuit boards, connectors, coil bobbins, bushings, condenser films and alkali battery cases), automobile parts (e.g., battery caps, fuses, sensors and ignitions), and other industrial products (e.g., ultrafiltration modules, chemical pumps and plating rolls).
It is also used for coating materials, paints, adhesive agents and films with its excellent characteristics described above. When these products are produced, normally a polysulfone-based resin is dissolved in a solvent to prepare the solution composition, which is dried either directly or after being spread on a base surface.
The representative products include a heat-resistant optical film used for producing transparent electrode bases for liquid crystal displays.
A base of glass has been employed as a transparent electrode base for liquid crystal displays, because of its excellent optical characteristics and very high surface smoothness, when ground. However, a glass board is high in density and should be itself sufficiently thick because of its fragility. Therefore, it is difficult to make a liquid crystal display on a glass base compact, light and resistant to impact. Use of a high-molecular-weight film has been proposed as a method to solve the disadvantages of the devices which include a glass base (e.g., Japanese Patent Laid-open Publication No.53-68099 and Japanese Patent Laid-open Publication No.54-126559).
Japanese Patent Laid-open Publication No.53-68099 and Japanese Patent Laid-open Publication No.54-126559 disclose continuous production of liquid crystal displays using a long polyester film coated with an electroconductive oxidized metallic material by evaporation, in place of a glass base. However, no polyester film having as smooth a surface as that of glass base has been produced.
In particular, when a liquid crystal display is made super twisted nematic (STN) to produce highly precise images, surface smoothness of high-molecular-weight film is very important, because the image is displayed using birefringence of liquid crystal between bases arranged at controlled intervals of the order of 0.1 μm.
An optical plastic laminated sheet excellent in heat resistance and transparency has been proposed as the one which can solve the problems involved in the polyester film, as disclosed by, e.g., Japanese Patent Laid-open Publication No.49-110725. It is composed of at least one layer of polysulfone-based resin (first layer) and second layer of optically transparent material (polysulfone-based resin) having a glass transition temperature lower than that of the first layer.
The laminated sheet disclosed by the patent document, although technically improved in that it includes polysulfone-based resin, which has higher heat resistance and better optical characteristics than the plastic which constitutes each layer, is produced by melt coextrusion, or lamination of each layer individually produced by solvent casting or melt extrusion to have a film of desired thickness. However, the laminated sheet produced by the above procedure has insufficient surface smoothness.
For production of a solution composition of polysulfone-based resin, on the other hand, use of a mixed solvent of strongly acidic, inert liquids of dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) as the main components, and alicyclic ketone and highly volatile aliphatic ketone is proposed by, e.g., Japanese Patent Laid-open Publication No.49-110725.
However, the above solution composition involves problems of poor smoothness of the coating surface, evaporation while it is being spread and dried, and generation of harmful components, e.g., SOx and NOx while it is combusted for disposal.